Capacitors are used to store electrical charge or energy, which stored energy can then be instantaneously released when needed. Capacitors can also function as filters by passing alternating current (AC) while blocking direct current (DC), and are also useful to prevent current and voltage transients.
Typically, a capacitor comprises two electrodes or electrode plates facing each other, with an insulating dielectric between them. The electrodes are made of a highly conductive material (often metal), and the dielectric is often made of a polymer material. Very thin polymer film capacitors are widely used, and are particularly useful in high voltage applications having space constraints. An ongoing challenge with these capacitors is slowing the rate at which they age and, thus, extending the useful life of the capacitors. With a greater understanding of the aging process undergone by thin film polymer capacitors, significant improvements have been made in their construction; namely, a move away from aluminum foil electrodes to electrodes formed by vapor deposition of very thin metal films (i.e., on the order of 20 to 100 nm), usually made of aluminum, gold or zinc, onto the polymer film. See, e.g., Reed et al., “The Fundamentals of Aging in HV Polymer-film Capacitors,” IEEE Transactions on Dielectrics and Electrical Insulation, Vol. 1 No. 5, October 1994, incorporated herein by reference.
Metallized thin film capacitors can be constructed so as to exhibit a “self-healing” ability (also referred to as “self-clearing”), i.e., the ability to recover from dielectric breakdown events. Dielectric breakdown is due to either extrinsic effects (e.g., material defects or porosity) or intrinsic effects (e.g., thermal runaway). When breakdown occurs, an electrical arc causes a short circuit between the metalized electrodes of the capacitor, and there is an in-rush of current through the fault in the dielectric material. This shorting event occurs very quickly, at a rate in the range of nano- or micro-seconds. (Such a defect within the capacitor and the resulting dielectric breakdown is schematically illustrated in the first two frames of FIG. 1.) Without self-healing capability, a single dielectric breakdown results in catastrophic failure and the entire capacitor can no longer hold or store a charge. For many applications, this failure is harmful to other components in the circuit and can result in catastrophic damage to the entire circuit. At worst, the localized heating that occurs at a short circuit may ignite the dielectric material, causing the circuit to burn up.
With capacitors having so-called self-healing properties, catastrophic failures are obviated. When a short circuit occurs as described above, the current therethrough vaporizes small areas of the metallized layer surrounding the short circuit thereby interrupting the flow of current (schematically illustrated in the last frame of FIG. 1) and returning the capacitor to normal function, usually with only a small reduction in capacitance. Thus, the self-healing process allows multiple isolated failure events to occur within the capacitor before an appreciable loss of capacitance is experienced. This confers better circuit stability, and extends the useful life of the capacitor and the circuit in which it is employed. The typical expiration of these types of capacitors is not catastrophic, but rather, is gradual due to the progressive loss of electrode material from repeated “self-clearing” events which eventually result in an open circuit condition.
This self-clearing process occurs very quickly in metallized film capacitors, typically on the order of nanoseconds to microseconds, at which rate, any disruption in capacitor performance is nominal. For such “fast clearing” to occur, the electrode material must be highly conductive. High conductivity enables high current densities at the fault which in turn causes the electrode to heat rapidly. Another requirement for fast clearing to take place is that the electrode material must be able to be made thin enough to vaporize and/or oxidize. Metals such as aluminum and gold fit this profile, and as such, are commonly used to metalize thin film dielectrics.
Irrespective of the benefits of self-healing properties, the cumulative effect of repeated clearing events results in a progressive loss of capacitance in self-healing capacitors. As such, an alternative or additional approach to extending the life of metallized capacitors has been developed. This approach involves segmenting the metallized films. See, e.g., U.S. Pat. Nos. 4,433,359, 5,717,563 and 6,631,068. Such segmentation involves forming a non-metallized pattern within the metal film to form a plurality of metallized segments. The non-metallized pattern provides unmetalized margins that interlink the metallized segment by small fuses. If a short circuit occurs within a segment, the fuses that interlink that segment to adjacent segments open and remove the segment from capacitor, thereby avoiding a catastrophic failure of the capacitor and the circuit in which the capacitor is used.
A significant downside to this approach, however, is that it requires a high degree of redundancy to be built into the device, thereby increasing the likelihood of reliability problems. Additionally, segmentation of the electrode can significantly increase the cost to fabricate the capacitors due to the loss of material involved and the additional process steps necessary to segment the electrodes.
Still yet, improvements to self-clearing and segmentation of thin metal electrodes which minimize the aforementioned disadvantages continue to be made. See, e.g., U.S. Pat. Nos. 6,631,068 and 7,099,141. These improvements, however, have thus far been limited to relatively stiff or rigid metallized films, and have not been applied to complaint or stretchable film materials. In applications such as dielectric elastomer actuators, in which capacitive structures are subject to actuation strains greater than about 5%, flat metallized films will typically crack within just a few actuation cycles. Even if the cracks are initially short and disconnected, for example in the case of sputtered metal coatings, commercial applications commonly require millions of strain cycles, a regime that propagates and connects cracks thereby interrupting electrical continuity.
As such, the advent of dielectric elastomer materials, also referred to as “electroactive polymers” (EAPs) provides a true technological advancement. An EAP transducer comprises two thin film electrodes having elastic characteristics and separated by a thin elastomeric dielectric polymer. When a voltage difference is applied to the electrodes, the oppositely-charged electrodes attract each other thereby compressing the polymer dielectric layer therebetween. As the electrodes are pulled closer together, the dielectric polymer film becomes thinner (the z-axis component contracts) as it expands in the planar directions (the x- and y-axes components expand). Furthermore, the like (same) charge distributed across each elastic film electrode causes the conductive particles embedded within that electrode to repel one another, thereby contributing to the expansion of the elastic electrodes and dielectric films.
Especially for actuator and generator applications, a number of design considerations favor the selection and use of advanced electroactive polymer technology based transducers. These considerations include potential force, power density, power conversion/consumption, size, weight, cost, response time, duty cycle, service requirements, environmental impact, etc. Electroactive polymer technology excels in each of these categories relative to other available technologies and, as such, offers an ideal replacement for piezoelectric, shape-memory alloy (SMA) and electromagnetic devices such as motors and solenoids. Examples of EAP devices and their applications are described in U.S. Pat. Nos. 6,940,221; 6,911,764; 6,891,317; 6,882,086; 6,876,135; 6,812,624; 6,809,462; 6,806,621; 6,781,284; 6,768,246; 6,707,236; 6,664,718; 6,628,040; 6,586,859; 6,583,533; 6,545,384; 6,543,110; 6,376,971 and 6,343,129; and U.S. Published Patent Application Nos. 2005/0157893; 2004/0263028; 2007/0217671; 2004/0124738; 2004/0046739; 2004/0008853; 2003/0214199; 2002/0175598; and 2002/0122561, the entireties of which are incorporated herein by reference.
In light of the realized advantages of devices employing compliant or stretchable electroactive materials, the need for self-healing properties in these kinds of materials is imperative. The inventors of the subject invention are not aware of any prior art electroactive polymer actuators that exhibit repeatable self-healing and reliable long-term conductivity consistent with commercially viable product lifetimes. To the contrary—failure modes of prior art devices are unpredictable and catastrophic. Thus, it would be highly advantageous to fabricate and provide electroactive materials and devices of a compliant or stretchable thin film construct having self-healing properties which overcome the limitations of the prior art.